Diameter range is the most notable difference between collets and chucks. Generally collets are used on smaller work pieces than typical three-jaw chucks. The general range for collets are 1/16in to 2.5in whereas chucks can be used on work pieces that are larger.

The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet (which were a letter series of names) which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres.

Disadvantages of collets Collets are limited in the range of workpiece sizes they can accommodate and are ideally suited for repetitive jobs where the clamping requirements don't vary greatly. They are also better suited for clamping tooling with longer shanks and workpieces with consistent diameters.

The easiest way to determine what type of collet your machine has is to measure the length and diameter of the collet. All collets have a distinctive length and diameter.